adb devices

Android adb command for host, provides a command line if you want to access the device shell of Android. Once you installed Adb as mentioned in “How to Install ADB & Fastboot on Ubuntu” you will need to connect your Android Mobile over USB and type command,

$ adb devices
 List of devices attached
 daemon not running; starting now at tcp:5037
 daemon started successfully
 B2NGAA8831702707    device

here, if you have run adb devices command first time, it will start the adb server daemon and list the device attached over USB with device serial number and name.

When you connect next time, and check adb devices, you can see the demon running messages not shown and list the devices connected as,

$ adb devices

List of devices attached
B2NGAA8831702707	device

You can also check the device product, model, USB bus connected to as,

$ adb devices -l

List of devices attached
B2NGAA8831702707       device usb:1-3 product:Onyx_00WW model:Nokia_7_plus device:B2N_sprout transport_id:56

Now, you can connect to this device using adb shell as,

$ adb shell

You can also connect to adb shell using serial number as,

 $ adb -s B2NGAA8831702707 shell 

Refer For “How to use adb shell when multiple devices are connected“